1. Field of the Invention
The present invention relates, in general, to methods and systems of communication networks, especially methods for efficiently transmitting medium access control (MAC) header data by devices of the network. More particularly, the invention relates to (but is not limited to) wireless communication networks operating in the Sub-1 GHz band, especially networks using the emerging IEEE standard 802.11ah.
2. Relevant Background
Many types of communication networks, both wired and wireless, transmit and receive packets of data by organizing transmitted signals into frames for coordination, synchronization and relaying of the transmissions. Ethernet is an example of a wired protocol for such signaling; examples of wireless networks include systems using the 802.11 protocols. One advantage to frames is to allow multiple devices to access to the same physical medium. But a disadvantage is that extra information, such as intended recipient, frame type, etc., must also be transmitted with the desired end user data in order to accomplish the coordination, synchronization and relaying. In many cases, such as with Ethernet and current 802.11 systems, this overhead information imposes a relatively light burden because data carrying frames often carry quite a bit of data, and the system uses a high proportion of such data carrying frames.
But in some systems which transmit relatively small amounts of data, on a less frequent basis, on channels (wired or wireless) with limited bandwidth, the overhead information can impose a large burden. An example of such a system is a wireless sensor network, with a large number of sensors. An IEEE 802.11 BSS or Wi-Fi system typically has a central device to communicate with perhaps tens of nearby users, each needing large data volumes (e.g., for viewing video, web pages, etc.) over time periods of seconds or less. In contrast, a sensor network could have hundreds, perhaps thousands, of widely dispersed sensors, each needing to send or receive small amounts of data to a central device, within time periods of minutes or even days. The amount overhead information needed to be sent with each frame, just for correct addressing of so many users, could seriously degrade the sensor network's capabilities.
To address this problem, as well as for efficient use of the radio spectrum, the Institute of Electrical and Electronic Engineers (IEEE) created Task Group ah (TGah) to develop standards so that wireless networks can transmit in a frequency band of 902 MHz to 928 MHz, called the Sub-1 GHz band. An advantage of this band is that greater range can be achieved. Also, there is typically less interference from intervening objects.
A third advantage of the Sub-1 GHz band is that no legacy systems with different protocols need to be accommodated. So communication systems and devices for this band can be designed to optimize overhead efficiency, rather than to optimize interoperability. In particular, the overhead information included in a transmitted frame can be reduced. In frame-based communication systems, the actual data packet to be transmitted to the receiving station and end user, called the payload, is included with other needed information, called header data. The header data allows the radio receiver to find the start of the frame, to determine the addressee of the payload, to check for errors, and to perform other system operations. Current standard communication protocols specify how the frames are to be structured into fields, which are often further structured with subfields. Also, for effective coordination of the system, some frames are designed only to send information for control and coordination of the transmissions, such as scheduling of transmission times by the various system devices. For example, in the 802.11 standards, there are three types of frames: control, data and management. The detailed terminology of frames and frame-based communications are specified in the standard IEEE 802.11-2012. The standard is cited as a reference for terminology and background information about frame transmission, and does not imply that the communication networks of this disclosure necessarily use the physical wireless transmission methods described therein.
FIG. 1 shows standard arrangements of data and management frames known from the IEEE 802.11a/b/g/n standards. In a data frame, the header consists of up to ten fields with a total size of 36+4 octets (or bytes). The Frame Control field conveys information on signaling and the type of frame being sent. Typically three MAC address fields, and sometimes four, are needed to distinguish the source device of the data, the data's destination device, and possible intermediate transmitter and receiver devices. The QoS, HT Control, Duration/ID fields convey information for coordinating channel access among the devices in the network. Finally, a Frame Check Sequence field typically includes the bits of a Cyclic Redundancy Check code used to ensure the frame header fields have been received correctly. The header of a management frame comprises many of the same fields, and three MAC address fields. The current inventions implement methods and systems for reducing this inefficiency.